Well as some people mentionned, he probably started this alone to make some money and didn't realize how big it would get. Then he probably needed a bigger team and it probably evolved over time and he eventually got bored of keeping Minecraft and other programs up to date. Also selling the company to a big entity was probably pretty attracting in terms of sales price. I don't know for how much he sold Mojang, but just because of Minecraft alone it must have been pretty lucrative. The main concern is what Microsoft will do with Minecraft and the third-party mods after its sale:

-Will Minecraft retail price be increased based on popularity? (For example, launch prices are $59.99 in North America and popular games remain between $39.99 and $59.99 for years, while Minecraft always retailed for $19.99 or less and I'm unsure if Minecraft can be considered a mini-game that is usually sold on PSN/Wiiware for $4-8 like other independent games). And I did see older game prices go up in the past: Dragon Crown PS3 and Shin Megami Tensei IV 3DS are notable examples. Granted, Microsoft prices are not as bad as they were a few years ago (I still remember seeing Office 2000 for $499.99 in Staples flyers on a regular basis a decade ago and now they high-end copy of Office 2013 is like $250 max)

-Will Microsoft be open towards the modding community as Notch was or will they be like Apple?

-Will they still develop the game for rival gaming consoles?